Machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy machinery utilize power sources to accomplish tasks including motion of the machine and operation of an implement driven by an implement pump. These power sources have limited rotational speed ranges measured in rotations per minute (RPM) and are often linked to one or more power conversion units, which may in turn be operatively connected to one or more driven traction devices with the goal of imparting motion to the machine through use of the rotational energy.
However, once a machine is underway, the machine may develop significant momentum based on the weight and velocity of the machine, among other things (e.g., potential energy associated with elevation of the machine). Where it is desired to decelerate the machine, mechanical braking (e.g., disc and/or drum type brakes) may be used in conjunction with some level of engine braking. Engine braking involves the use of energy dependent compression strokes of the power source, frictional losses, and ancillary mechanisms to dissipate energy and decelerate the machine. When using engine braking, some of the momentum associated with the machine is transferred back into rotational energy and transmitted through driven traction devices, a power conversion unit, and ultimately to the power source resulting in an increase in rotational speed of the power source.
Dependence on mechanical braking can lead to overheating and premature wear on the braking system. Engine braking may lead to exceeding the rotational speed constraints of the power source (i.e., over-speeding) and therefore, may cause premature wear and/or damage to the power source. Therefore, in previous systems, limitation of mechanical braking and the over-speeding condition have been accomplished via use of a fluid coupling (i.e., a torque converter) between the power source and the power conversion unit. This fluid coupling may absorb a portion of the momentum associated with the machine and convert it into fluid heat energy and/or rotational energy, but may be insufficient to fully retard the motion of the machine. Further recent changes to power conversion units have, in some cases, eliminated the fluid coupling between the power source and the power conversion unit, thus resulting in even greater potential for over-speed of the power source and substantial reliance on mechanical braking systems. Such reliance may lead to substantial wear and early failure of components associated with the power source and mechanical braking systems.
One system describing an over-speed protection control for an engine is disclosed in U.S. Pat. No. 3,999,386 (the '386 patent) issued to Crull et al. on Dec. 28, 1976. The system includes at least one fixed-displacement pump driven by an engine and having a valve means operable to cause an increase in pressure associated with the pump in response to an engine speed exceeding a predetermined value. Such an increase in pressure thereby increases the load on the engine and affects a retarding of the engine. The system further includes a control circuit for detecting when the engine speed exceeds the predetermined value.
Although the system of the '386 patent may retard the engine upon an over-speed condition, the '386 patent is limited in that the control occurs only upon an over-speed event, thus leading to greater reliance on mechanical braking systems unless the over-speed condition occurs. Further, the system of the '386 patent is in operation continuously. In other words at any time an over-speed condition has been met, the system of the '386 patent causes a retarding of the machine, thereby leading to potential degradation in machine performance when it is desired to increase the engine speed beyond the predetermined value even for short periods of time.
The present disclosure is directed to overcoming one or more of the problems or disadvantages in the prior art control systems.